Field of the Invention
The present specification relates to an ignition plug for igniting fuel gas in, for example, an internal combustion engine.
Description of the Related Art
Regarding ignition plugs, a technology of joining an electrode tip to an electrode body in order to increase the durability of the electrode is known (see, for example, PTL 1). The electrode tip is made of a material that is more durable with respect to spark discharge and oxidation than the electrode body. Examples of the material include a noble metal (such as platinum, iridium, ruthenium, and rhodium) and an alloy containing a noble metal as a main component. Since the electrode body and the electrode tip are joined to each other by using various methods, such as laser welding and resistance welding, a welding portion is formed between the electrode body and the electrode tip.
When an ignition plug is used in an internal combustion engine, thermal stress occurs in the welding portion due to combustion heat. Therefore, cracks tend to occur at a boundary between the electrode tip and the welding portion and at a boundary between the electrode body and the welding portion. When such cracks occur at these boundaries, the electrode tip may be peeled off from the electrode body.